Multi-threshold voltage IC devices are often utilized in the semiconductor integrated circuit (IC) industry to optimize delay or power. A multi-threshold voltage IC device may include several different devices, each having a different threshold voltage (i.e., operating voltage). For example, the multi-threshold voltage IC device may include a low threshold voltage device and a high threshold voltage device. One approach to achieving the different threshold voltage devices includes channel and halo implantation optimization. This implements heavy implantations to achieve the higher threshold voltage devices, and separate masks for each desired threshold voltage. It has been observed that the heavier implantation processes can cause mobility degradation and junction leakage current, and using a separate mask for each desired threshold voltage incurs unnecessary cost. For example, band-to-band tunneling current is high under heavy implantation such that the OFF current is increased. Carrier mobility degradation decreases the ON current.
Another approach, which has been used in conjunction with the channel/halo implantation optimization, includes enlarging a channel length of the higher threshold voltage devices. However, as technology nodes continue to decrease, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. Enlarging channel lengths to accommodate higher threshold voltage devices thus consumes valuable IC device space, limiting the amount of devices that can be fabricated on a single chip. Furthermore, longer channel length reduces the ON current as well as increases capacitance, hence reducing the speed of device operation.
Accordingly, although existing approaches for fabricating multi-threshold voltage devices have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects